1. Field of the Invention
The present invention relates to image-forming optical systems. More particularly, the present invention relates to a decentered optical system with a reflecting surface having a power for use in optical apparatus using a small-sized image pickup device, e.g. video cameras, digital still cameras, film scanners, and endoscopes.
2. Discussion of Related Art
Recently, with the achievement of small-sized image pickup devices, image-forming optical systems for use in video cameras, digital still cameras, film scanners, endoscopes, etc. have also been demanded to be reduced in size and weight and also in cost. Further, there have recently been commercially available portable telephones, PDAs and notebook personal computers that have built-in electronic image pickup optical systems. Accordingly, there are strong demands that such optical systems should be further reduced in thickness.
Under these circumstances, there have recently been proposed optical systems designed to be compact and thin by giving a power to a reflecting surface, which produces no chromatic aberration, and folding an optical path in the optical axis direction.
Japanese Patent Application Unexamined Publication Numbers (hereinafter referred to as “JP(A)”) Hei 8-292371, 9-90229 and 10-68884 each disclose an optical system in which an optical path is folded by a single prism or a plurality of mirrors integrated into a single block, and an image is relayed in the optical system to form a final image.
In these conventional examples, however, the number of reflections increases because the image is relayed. Accordingly, surface accuracy errors and decentration accuracy errors are transferred while being added up. Consequently, the accuracy required for each surface becomes tight, causing the cost to increase unfavorably.
JP(A) Hei 9-211331 discloses an example of an optical system in which no relay image is formed. In this example, an optical path is folded by using a single prism to achieve a reduction in size of the optical system. However, the optical system has an extremely narrow photographic field angle and is not satisfactorily corrected for aberrations.
JP(A) Hei 10-20196 discloses a zoom lens system as an example of an optical system using two prisms, in which no relay image is formed. The zoom lens system comprises two units, i.e. a positive front unit and a negative rear unit. The positive front unit includes a prism of negative power placed on the object side of a stop and a prism of positive power placed on the image side of the stop. JP(A) Hei 10-20196 also discloses an example in which the positive front unit, which comprises a prism of negative power and a prism of positive power, is divided into two to form a three-unit zoom lens system having a negative unit, a positive unit and a negative unit. In these examples, however, the zoom lens systems are not approximately telecentric on the image side and hence incapable of being applied to optical systems using image pickup devices such as CCDs. Further, because the two prisms have a total of four reflecting surfaces, manufacturing tolerances are tightened, causing costs to increase unfavorably. Further, if two reflecting surfaces are used in the prism positioned on the object side of the aperture stop, the height of extra-axial rays becomes high, causing the prism to become large in size undesirably.
JP(A) 2000-111800 also discloses lens systems each using two prisms. However, these lens systems are not satisfactorily thin and compact. Further, an image pickup device (image plane) is not placed at right angles to the photographing direction. All things considered, including the substrate of the image pickup device, it is difficult to form a very thin image pickup unit.
When a general refracting optical system is used to obtain a desired refracting power, chromatic aberration occurs at an interface surface thereof according to chromatic dispersion characteristics of an optical element. To correct the chromatic aberration and also correct other ray aberrations, the refracting optical system needs a large number of constituent elements, causing costs to increase. In addition, because the optical path extends straight along the optical axis, the entire optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large-sized image pickup apparatus.
In decentered optical systems such as those described above in regard to the prior art, an imaged figure or the like is undesirably distorted and the correct shape cannot be reproduced unless the formed image is favorably corrected for aberrations, particularly rotationally asymmetric distortion.
Furthermore, in a case where a reflecting surface is used in a decentered optical system, the sensitivity to decentration errors of the reflecting surface is several times as high as that in the case of a refracting surface, and as the number of reflections increases, decentration errors that are transferred while being added up increase correspondingly. Consequently, manufacturing accuracy and assembly accuracy, e.g. surface accuracy and decentration accuracy, required for reflecting surfaces become even more strict.